Along with the development of the economy, in order to raise the output of the products and to gain more profit, the labor-intensive industry has moved to equipment-intensive one. After the industrial revolution, to produce products, the electricity becomes the main power source. The way to acquire the electricity also becomes the international main concern. Compared with the contaminating energy such as the petroleum, the coal, and the nuclear energy, the solar energy is a kind of energy which makes no pollution, provides the energy of equivalent 180 watts per meter square to the surface of the earth, and has no problem with the energy monopolization. Therefore, the solar energy has become one of the most potential energy in the future.
Since the first solar cell produced in Bell's laboratory in the United States in 1954, various kinds of solar cells with different structures have been disclosed afterwards. The solar cells can be classified as the silicon-based solar cell, the multi junction semiconductor solar cell, the dye sensitized solar cell, and the organic conductive polymer solar cell and so on in accordance with the difference of the materials. In accordance with FIG. 1, take the conventional silicon-based solar cell device 1 for example, the structure comprises a first electrode 12, a silicon substrate 17, a p-type silicon semiconductor layer 14, an n-type semiconductor layer 15, and a second electrode 16. The sun light 10 illuminates the solar cell device 1 and provides the p-type silicon semiconductor layer 14 and the n-type semiconductor layer 15 the energy which is larger than the band gap of the Si semiconductor layer. After the atoms in the silicon semiconductor layer absorbing the energy, the free carriers (electrons/holes) are produced. The produced electrons move toward the n-type semiconductor layer 15, the produced holes move toward the p-type semiconductor layer 14, and the electric potential difference is produced because the positive and the negative charges accumulate near the p-n junction between the p-type silicon semiconductor layer 14 and the n-type semiconductor layer 15. Due to the electric potential difference, the accumulated electrons flow to the external circuit (not shown in the figures) from the first electrode 12 to the second electrode 16 to generate the current in the external circuit. Meanwhile, if a load (not shown in the figures) is added in the external circuit, the produced electric energy can be collected and stored. Herein, the combination of the p-type silicon semiconductor layer 14 and the n-type semiconductor layer 15 which can absorb a light with a specified wavelength range and produce a current in the external circuit is also called an optical-electric conversion layer 11.
FIG. 2 shows the spectrum of the solar energy radiation on the surface of the earth. In accordance with the spectrum, the distribution of the solar energy on the surface of the earth, besides the visible light, the IR and the ultraviolet light also distribute. Nevertheless, due to the basic mechanism of the solar cell as mentioned above, only the solar energy equal to or larger than the band gap of the semiconductor layer can be absorbed in the traditional semiconductor solar cell structure. Take silicon for example, the band gap of silicon is about 1.12 eV, so it can absorb only part of the energy with the wavelength in the IR range in the spectrum. Besides, in consideration of the internal loss of the solar cell, the low conversion efficiency of the solar cell is indeed a problem.
In order to improve the aforementioned problem, a multi junction solar cell is developed and has become one of the solar cell structures with the highest conversion efficiency.
Refer to FIG. 3, 3 is a kind of the multi-junction solar cell device, which comprises a three optical-electric conversion layers (p-n junctions) of Ge/Ga1−yInyAs/Ga1−xInxP inside the device. The multi junction solar cell device 3 comprises a first electrode 32, a Ge substrate 35, a first optical-electric conversion layer 31 composed of Ge, a second optical-electric conversion layer 33 composed of Ga1−yInyAs, a third optical-electric conversion layer 34 composed of Ga1−xInxP, and a second electrode 36. Each optical-electric conversion layer is a p-n junction formed by the combination of one p-type semiconductor layer and one n-type semiconductor layer. Accordingly, the first Ge optical-electric conversion layer 31 is a p-n junction formed by the combination of a p-type Ge semiconductor layer 311 (p-Ge) and an n-type Ge semiconductor layer 312 (n-Ge); the second Ga1−yInyAs optical-electric conversion layer 33 is a p-n junction formed by the combination of a p-type Ga1−yInyAs semiconductor layer 331 (p-Ga1−yInyAs) and an n-type Ga1−yInyAs semiconductor layer 332 (n-Ga1−yInyAs); the third Ga1−xInxP optical-electric conversion layer 34 is a p-n junction formed by the combination of a p-type Ga1−xInxP semiconductor layer 341 (p-Ga1−xInxP) and an n-type Ga1−xInxP semiconductor layer 342 (n-Ga1−xInxP). When the sun light 30 illuminates, in order to let the aforementioned multi optical-electric conversion layers absorb the solar energy efficiently, the optical-electric conversion layer nearest the sun is preferably a layer with the larger semiconductor band gap wherein the band gap decreases gradually to the desired band gap of the optical-electric conversion layers. Accordingly, the band gap of the Ga1−xInxP optical-electric conversion layer 34 is larger than the band gap of the Ga1−yInyAs optical-electric conversion layer 33, and the band gap of the Ga1−yInyAs optical-electric conversion layer 33 is larger than the band gap of the Ge optical-electric conversion layer 31.
Besides, there is a first tunnel junction 38 between the first optical-electric conversion layer 31 and the second optical-electric conversion layer 33, and a second tunnel junction 39 between the second optical-electric conversion layer 33 and the third optical-electric conversion layer 34. The tunnel junctions locate between the optical-electric conversion layers to adjust the resistance between two adjacent optical-electric conversion layers, to reduce the charges accumulated near any side of the two adjacent optical-electric conversion layers, and to make the currents of the optical-electric conversion layers consistent.
When the sun light 30 passes through the upper Ga1−xInxP optical-electric conversion layer 34 with higher band gap, the photon with higher energy is absorbed (Ga1−xInxP(1.85 eV; x˜0.5), the absorbed spectrum is about from the ultraviolet to the visible light), and then, the central Ga1−yInyAs optical-electric conversion layer 33 (Ga1−yInyAs(y˜0.01)) absorbs the photon with the energy from the visible light to the IR part because its band gap is smaller than that of the Ga1−xInxP optical-electric conversion layer. It also re-absorbs the higher energy light which is not absorbed by the upper Ga1−xInxP optical-electric conversion layer 34 transmitting from the upper layer to this central layer and to recycle the solar energy more efficiently. Finally, because the Ge optical-electric conversion layer 31 comprises the lowest band gap, it can re-absorb the light with the energy larger than the IR light passing through the upper two layers. Referring to FIG. 4, FIG. 4 shows the spectrum response diagram of the multi junction solar cell device 3. One coordinate axis shows the absorbed wavelength and the other coordinate axis shows the percentage of the quantum efficiency. The higher the quantum efficiency is, the more efficiently the selected material absorbs the light with the corresponding wavelength and converts it into the electron-hole pairs in the solar cell. As shown in FIG. 4, because the multi junction solar cell with gradually increased band gaps from the substrate to the solar cell of the Ge/Ga1−yInyAs/Ga1−xInxP composition comprises a broader and overlapping absorbing wavelength range, the solar energy can be used repeatedly and the solar cell can achieve the extremely high quantum efficiency. Therefore, such stacked multi junction solar cell has higher conversion efficiency.
However, the design of one multi-junction solar cell is not only depends on the match of the band gaps between the different optical-electric conversion layers, the current balance also should be achieved by adjusting the thicknesses of the different materials of the optical-electric conversion layers. Besides, the lattice constants of the materials of the optical-electric conversion layers also should be matched to reduce the defects of the solar cell during the producing process in order to improve the quality and the conversion efficiency of the solar cell devices.
Referring to FIG. 3, the main structures of the solar cell device 3 from the bottom are the Ge substrate 35, the Ge optical-electric conversion layer 31, the Ga1−yInyAs optical-electric conversion layer 33, and the Ga1−xInxP optical-electric conversion layer 34. The lattice constant of the Ge substrate 35 and the Ge optical-electric conversion layer 31 is about 5.66 A, the lattice constant of the Ga1−yInyAs optical-electric conversion layer 33 is about 5.64 A, and the lattice constant of the Ga1−xInxP optical-electric conversion layer 34 is also about 5.64 A. Therefore, to the Ge substrate 35, the lattice constants of the Ga1−yInyAs optical-electric conversion layer 33 and the Ga1−xInxP optical-electric conversion layer 34 are smaller, and the Ge substrate 35 provides tensile stresses to the Ga1−yInyAs optical-electric conversion layer 33 and the Ga1−xInxP optical-electric conversion layer 34. On the whole, the optical-electric conversion layers on the Ge substrate comprise the lattice constants equal to or smaller than the lattice constant of the Ge substrate 35, which means the upper stacks are effected by the tensile stress from the Ge substrate 35 that can generate bending or cracks and influence the quality and the yield of the devices.